1. Field of the Invention
The present invention relates to a radiation image detecting device for detecting a radiation image.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using radiation such as X-rays is known. The X-ray imaging system includes an X-ray generating apparatus for generating X-rays and an X-ray imaging apparatus for acquiring an X-ray image of an object (i.e. patient) from the X-rays having passed through the object. The X-ray generating apparatus includes an X-ray source for irradiating the X-rays to the object, a source controller for controlling operation of the X-ray source, and an irradiation switch for inputting a command to actuate the X-ray source to the source controller. The X-ray imaging apparatus includes an X-ray image detecting device for detecting the X-ray image by converting the X-rays having passed through the object into an electrical signal, and a console for controlling operation of the X-ray image detecting device and storing and displaying the X-ray image.
An X-ray imaging apparatus using an X-ray image detecting device for electronically detecting an X-ray image has been widely spread instead of an X-ray image recording device using an X-ray film or an imaging plate (IP) cassette. The X-ray image detecting device has a sensor panel that is also referred to as a flat panel detector (FPD). The sensor panel has an imaging area in which a plurality of pixels each for accumulating signal charges corresponding to a dose of incident X-rays are arranged in a matrix. Each of the pixels has a photoelectric converter for generating electric charges and accumulating the generated electric charges, and a switching element such as a thin film transistor (TFT). The sensor panel reads out the signal charges accumulated in the photoelectric converter of each of the pixels through a signal line disposed for each column of the pixels to a signal processing circuit upon turning-on of the switching element. The signal charges are converted into a voltage signal in the signal processing circuit. Thereby, an X-ray image is electronically detected.
In the X-ray image detecting device using the sensor panel, a reset operation is periodically carried out by the sensor panel for the purpose of discharging unnecessary electric charges from the pixels so as to minimize influence of noise of dark current added to the X-ray image. Accordingly, in the X-ray imaging system including the X-ray image detecting device using the sensor panel, the timing to start X-ray irradiation is synchronized with the timing for the sensor panel to finish the reset operation and start an accumulation operation for accumulating the signal charges in the pixels. For example, the source controller and the X-ray image detecting device are respectively provided with an interface (I/F) to establish intercommunication therebetween. The source controller transmits a synchronizing signal to the X-ray image detecting device in accordance with the timing to start X-ray irradiation. Upon receiving the synchronizing signal, the X-ray image detecting device makes the sensor panel shift to the accumulation operation. Alternatively, there is the following X-ray image detecting device having a function of judging whether or not X-ray irradiation has been started. The X-ray image detecting device is not connected to the source controller, and does not exchange any synchronizing signal with the source controller. Instead, the X-ray image detecting device detects the X-ray dose by a dose detection sensor, compares the detected X-ray dose with a predetermined irradiation start threshold value, and judges that the X-ray irradiation has been started when the detected X-ray dose exceeds the irradiation start threshold value. Then, the X-ray image detecting device makes the sensor panel start the accumulation operation.
Further, in some cases, an automatic exposure control (AEC) is performed in the X-ray imaging system. According to the AEC, in order to not only obtain an X-ray image having an appropriate image quality but also reduce X-ray exposure to an object, the X-ray dose is detected by a dose detection sensor during the X-ray imaging (during the X-ray irradiation), and the X-ray irradiation from the X-ray source is stopped when an integrated value of the dose (accumulated dose) reaches a target dose. The dose of X-rays irradiated from the X-ray source is determined by a tube current-time product (mAs value) as a product of X-ray irradiation time and a tube current for defining the dose of X-rays irradiated from the X-ray source per unit time. There are approximate recommended values for the items of imaging conditions including the irradiation time and the tube current depending on a body part of the object to be imaged (such as chest and head), the sex and age of the object, and the like. However, since X-ray transmittance is varied depending on an individual difference such as body frame of the object, the AEC is performed to achieve more appropriate image quality.
Conventionally, an ion chamber or the like has been used as a dose detection sensor. However, recently, there is proposed a technique of subjecting pixels in a sensor panel to simple modification such that the pixels serve as dose detection sensors. According to United States Patent Application Publication No. 2011/0180717 (corresponding to Japanese Patent Laid-Open Publication No. 2011-174908), some of pixels are used as dose detection sensors. Each of the pixels used as the dose detection sensor (hereinafter referred to as detection pixel) is connected to the wiring for radiation detection without through a switching element. Therefore, irrespective of whether the switching element is turned on or turned off, an output corresponding to the electric charges generated in the detection pixel (hereinafter referred to as dose signal) flow into the wiring for radiation detection. The dose signal is sampled at a predetermined cycle in a signal processing circuit to which the wiring for radiation detection is connected. Then, the dose signal is inputted to a control section. In the control section, based on the dose signal, the judgment on whether or not the X-ray irradiation has been started, or the AEC is performed. The signal processing circuit has an amplifier so as to amplify the dose signal flowing from the wiring for radiation detection at the time of judging whether or not the X-ray irradiation has been started, and outputs the amplified dose signal to the control section.
According to an irradiation profile representing change in the dose of X-rays per unit of time with the passage of time, there is a rising period during which the dose of X-rays gradually increases immediately after the start of X-ray irradiation. After the rising period, the dose of X-rays achieves a value corresponding to a tube current set by the imaging condition, and then the X-ray irradiation is continued at the dose of X-rays in a stationary period. During the AEC, the sampling operation of the dose signal is continued in the stationary period, and the accumulated dose is compared with the target dose based on the sampled dose signal, so as to determine the timing to stop the X-ray irradiation.
In contrast, the judgment on whether or not the X-ray irradiation has been started is required to be made promptly in order to minimize waste of the irradiated X-rays. Therefore, it is late to judge whether or not the X-ray irradiation has been started after the stationary period starts, and it is necessary to judge whether or not the X-ray irradiation has been started based on the dose signal which has been sampled during the rising period. Namely, the sampling period of the dose signal in the case of judging whether or not the X-ray irradiation has been started is much shorter than the sampling period of the dose signal in the case of performing the AEC.
It is known that random noise is generated in an analog signal processing circuit for reading out a dose signal. With regard to the random noise, there are random noise as positive components for increasing a signal value of the dose signal, and random noise as negative components for decreasing the signal value of the dose signal. In the AEC, the random noise as the positive components and the random noise as the negative components cancel each other in the course of integrating the dose signals obtained plural times in the sampling period longer than the sampling period of the dose signal in the case of judging whether or not the X-ray irradiation has been started. As a result, the noise components in the amplification gain at the time of reading out the dose signal are cancelled out, and thus the amplification gain contributes to only the amplification of the signal components. Consequently, as the gain is higher, the dose signal having a higher signal-noise ratio (S/N ratio) can be obtained, and the AEC can be performed at high precision.
In contrast, in the case where the judgment on whether or not the X-ray irradiation has been started is made, the sampling period of the dose signal is short. Therefore, the degree of cancellation between the positive components and the negative components in the random noise is smaller in comparison with the case where the AEC is performed. Accordingly, in the case where the gain to be applied to the dose signal is set to be equal to or higher than that in the case of performing the AEC, the random noise becomes large, and the S/N ratio of the dose signal is decreased in some cases. The problem becomes more serious as the sampling period is made shorter for the purpose of ensuring promptness in the judgment on whether or not the X-ray irradiation has been started. In the case where the sampling period is made shorter, the dose signal obtained at the first sampling is compared with the irradiation start threshold value in an extreme example, and the size of the random noise directly affects the S/N ratio of the dose signal. In the case where the judgment on whether or not the X-ray irradiation has been started is made in a state that the S/N ratio of the dose signal is poor, the probability of erroneous judgment on whether or not the X-ray irradiation has been started naturally becomes high.